The present invention relates to a wind power generation system which can realize, when a voltage drop such as a power failure occurs in an electric power system, protection of a converter connected to the rotor AC excitation type generator by absorbing an overcurrent generated in the rotor.
In the AC excitation type generator used in a generating set, the frequency of a voltage generated in the stator as the rotor undergoes excitation can be made to be equal to a system frequency by AC exciting the rotor winding at a slip frequency (the difference between the system frequency and the rotating frequency) by means of a power converter. Making the excitation frequency of rotor (slip frequency) variable brings about such advantages that the number of rotations of a wind turbine can be allowed to be variable and the capacity of the power converter can be smaller than that of the generator.
In the event that a voltage drop due to, for example, a ground fault takes place in the electric power system, however, the AC excitation type generator is so operated as to supply a current to the fault point. At that time, an overcurrent is induced in the rotor winding, causing an excessive current to flow in a converter for excitation connected to the rotor and therefore, a countermeasure has been taken which installs a unit called a crow-bar for short-circuiting the rotor circuit by using thyristors.
Recently, standards for such a regulation that during a system fault, the wind power generation system should continue operation without being disconnected from the power system have been established in Europe and accordingly, it has been demanded that during a voltage drop for a short period, the wind power generation system should not be disconnected from the power system and running for power generation should be resumed after recovery from the fault, thus minimizing the influence the fault has upon an electric power system.
In the past, the AC excitation type generator has principally been used for a large scale generation system such as installed in a pumping-up power station and in the event of a large scale of power failure in the system, a running method of short-circuiting the rotor circuit by means of a separately-excited device has been adopted.